upgrade to libpng-1.5.18

Change-Id: I500645d655fc7e7c850b09e47bece26e74197eb7
Reviewed-on: https://gerrit.libreoffice.org/9344
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>